February 12, 1857. W. R. GROVE, Esq., V.P., in the Chair. The following communications were read : : I. "On the Photography of the Moon." By WILLIAM CROOKES, Esq. Communicated by Professor STOKES, Sec. R.S. Received December 16, 1856. The subject of lunar photography is one which has engaged the attention of scientific men almost from the first announcement of the possibility of fixing the images in the camera. Owing to the extreme difficulty of satisfying all the conditions of the problem to be solved, there are few good photographs of the moon yet in existence. It was my good fortune in the autumn of 1855 to obtain several excellent pictures of this kind, and since these form the starting-point of the work which, by the assistance of a grant from the Donation Fund of the Royal Society, I have been pursuing during the greater part of the last year, a detailed account of the means employed for their production will not, I think, be considered out of place here. The telescope in which these pictures were taken is the magnificent equatorial at the Liverpool Observatory. This, together with all the resources of the establishment, was placed at my disposal by my kind friend Mr. Hartnup, to whom it is but due to state, that, were it not for the invaluable assistance afforded me by his sterling advice as well as steady hand, the results would not have been worth keeping. The mounting of the equatorial is quite unique; the polar axis and telescope together weigh about five tons, and whilst all parts are so truly and smoothly fitted that this enormous mass is moved equatorially by means of a small water-mill with such marvellous accuracy, VOL. VIII. 2 F that a star viewed through it appears absolutely stationary, its firmness is such that a hard blow against the side merely produces a scarcely perceptible momentary deflection. The object-glass is 8 inches in diameter, and has a sidereal focus of 12.5 feet-the diameter of the moon's image in this focus being about 1.35 inch. The eyepiece was removed, and in its place the body of a small camera was attached, so that the moon's image might fall upon the ground glass or sensitive film in the usual manner. Much labour had been saved me in finding the true actinic focus, by several photographers of Liverpool, who were working for some time on the same subject when the British Association met in that city in 1854. They found that the object-glass had been over-corrected for the actinic rays-the plate being required to be placed at a distance of 0.8 of an inch beyond the optical focus: a few experiments were sufficient to enable me to verify this result. During the time above referred to, and frequently since, Mr. Hartnup had taken many hundreds of pictures with chemicals recommended by various persons, but had not succeeded in obtaining a good negative at all, and not even a positive with a less exposure than from half a minute to a minute. As I succeeded in taking dense negatives in about four seconds, with the temperature of the room below freezing and the moon at a considerable distance from the meridian, and as I attribute the greater sensitiveness which I obtained to the great purity of the materials I employed, I think it right to give, after the mechanical arrangements are described, an account of the way in which these were prepared. The clockwork movement was only sufficient to follow the moon approximately when on the meridian, but as the pictures were nearly all taken when she was some distance past the meridian, and when consequently the declination and atmospheric refraction were changing rapidly, it was necessary, notwithstanding the short time required to take the pictures, to correct for the imperfect motion of the telescope. This was done by means of slow-motion screws attached to the right ascension and declination circles, which are each 4 feet in diameter. The finder had an eyepiece with a power of 200 applied to it, having cross wires in its field. The modus operandi of taking the picture was as follows:-The telescope having been moved until the moon's image was in the centre of the focusing glass, the water-mill was turned on and the dark slide containing the sensitive collodion plate was substituted for the ground glass. Mr. Hartnup then took his station at the finder, and, with a tangent rod in each hand, by a steady and continuous movement, kept the point of intersection of the cross wires stationary on one spot on the moon's surface. When the motion was most perfectly neutralized, I uncovered the sensitive plate at a given signal and exposed it, counting the seconds by means of a loud-ticking chronometer by my side. From the ease with which on my first attempt I could keep the cross wires of the finder fixed on one point of the moon by means of the tangent rods, I confidently believe that with the well-tutored hands and consummate skill which guided this noble instrument, the moon's image was as motionless on the collodion plate as it could have been were it a terrestrial object. The negatives which I obtained by these means were exquisitely beautiful, and so minute that I could not obtain paper with a sufficiently fine surface whereon to print copies which would do them justice. It was evident that they would bear magnifying several diameters and still remain sharply defined. The expense of carrying out this design here stopped me, when by the kind advice of Professor Wheatstone I applied to the Royal Society, whose munificence has so frequently been the cause of bringing to a successful termination investigations of the highest importance. A half-plate photographic combination of lenses, by Ross, was screwed the reverse way into a large sliding camera body 10 inches high by 11 inches wide, and capable of sliding from 18 inches to 3 feet long. At the end of the sliding body opposite to the lens, was a groove to admit either a focusing glass or a dark slide for the sensitive plate. A smaller camera body was screwed into the other end of the brasswork of the lens, having also a groove in front to admit of a sliding box capable of holding the small negatives. A reflector was placed in front of all, so arranged as to move in altitude round a centre, and, being fixed in any required position, to reflect the diffused light of the sky through the negative and lens parallel with the axis of the latter. Preliminary trials showed me that there was no good gained by magnifying the small pictures more than about 20 times, as after that the individual parts begin to get confused and indistinct; this magnifying cannot, however, be effected at once. In the small negatives the lights and shades are the reverse of what they are in nature, consequently a print on paper therefrom gives the light and shade correct. A photographic copy of a negative, however, produces a positive by transmitted light, and a print from this would have the shadows light and the light parts dark; consequently, in magnifying a negative with the intention of still producing a negative, an intermediate transmitted positive must first be taken, and this in its turn magnified, when it will produce a negative. The relative distances of the negative and focusing glass from the intermediate lens were so adjusted, that an image of the negative, enlarged to about two diameters, was thrown upon the ground glass, care being taken that the light from the sky was reflected parallel through the centres of the negative and lens by means of the mirror. The aperture of the lens was then stopped down to half an inch by means of a diaphragm, and the focus most carefully obtained by sliding the end of the large camera in or out. I found it necessary to verify this by experimental trials at different distances on each side of the observed focus, as it was difficult to judge accurately with the eye on the ground glass, owing to the roughness of the latter and the feebleness of the light. A picture, or rather many pictures, were now taken, and the one which by transmitted light most truthfully resembled a paper print from the small negative was reserved for further magnifying. This was effected absolutely in the same manner as the former: the negative being removed and the positive being placed in its stead, a further magnifying gave a large-sized negative. Although this process seems very simple, it is impossible to estimate the difficulties, unless by an actual repetition of the experiment, which I had to overcome before arriving at the beautiful result which I have the honour to lay before the Society. The double copying had a tendency to slightly exaggerate the effect of light and shade, and this could only be obviated by exposing the plates for such a time, that with the feeble light at my command it was verging on decomposition; particles of dust, too, seemed most pertinaciously to fix themselves on the prominent mountains, giving rise to craters where none should be; and even my finished pictures are not per |